1. Field of the Invention
The present invention relates to lithographic apparatus and methods.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed.
Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Before exposing radiation to the substrate through the patterning device, the substrate and the patterning device should be aligned by positioning the substrate, the patterning device or both. Also, other parts of a lithographic apparatus may be moveable and may need to be positioned at any time during a process of applying a pattern onto the substrate. Positioning should be done accurately and may require an accurate positioning system.
Positioning may be performed using a driving device, e.g. a motor, to which the object may be attached, or otherwise connected. An example of a driving device is a Lorentz actuator, i.e. an actuator including at least one magnet for inducing a magnetic field, and at least one coil, positionable and moveable in the magnetic field. Due to physical laws, when a current runs through the coil, a force is generated which is directed orthogonal to both the current and the magnetic field. Due to this force, the coil and the magnet may move relative to each other. Thus, a control current supplied to the actuator controls the force exerted on the object, and thus its movement, the object being connected to either the coil or the magnet.
The magnet may be any kind of magnet, e.g. an electromagnet or a permanent or natural magnet. The coil may include a number of windings of conductive wire, for example. To generate a force, a current should run through the coil and a magnetic field should be present. To vary the strength of the force, the current may be varied or the intensity of the magnetic field may be varied, for example by varying the current running through an electromagnet.
Usually, a position control system employing position feedback is included in a control system connected to the driving device, e.g. a Lorentz actuator, to improve the accuracy of the positioning. Due to characteristics of the actuator, the actual position of the object after movement may deviate from the target position. Therefore, the actual position of the object may be measured during and after movement and may be fed back to the positioning system. A difference between the target position and the actual position may then be compensated by repositioning the object.
However, before the object arrives at the target position, such a position control system may have needed time to compensate a number of erroneous movements and repositioning the object a number of times. While the object may arrive at the target position with the required accuracy, the time needed to arrive at the target position, i.e. the settling time, may be relatively long. In case of a lithographic apparatus, a long settling time reduces the throughput of the apparatus.